1. Field of the Invention
The present invention relates generally to automotive engines and more specifically to a fuel supply control system for an internal combustion engine of the type used in automotive vehicles and the like.
2. Description of the Prior Art
FIG. 1 schematically shows an internal combustion engine system 1 which includes an induction port 2, an induction manifold arrangement 3 which is operatively connected with the induction port; a throttle valve 4 disposed in the induction manifold arrangement for controlling the amount of air which is permitted to pass to the induction port 2; an air flow sensor disposed in the induction manifold at a location upstream of the throttle valve for sensing the amount air passing through the induction system; an engine speed sensor 6 for producing a train of pulses the frequency of which is indicative of the engine rotational speed. The system further includes a fuel injector arranged to inject fuel into the induction port 2; an exhaust manifold 8; an air-fuel ratio sensor disposed in the exhaust manifold and arranged to sense the amount of oxygen present in the exhaust gases; an engine temperature coolant sensor which projects into the coolant jacket of the engine; a spark plug 12 and a control circuit 13. The latter mentioned circuit is arranged to receive inputs from the air flow sensor 5, engine speed sensor 6, air-fuel ratio sensor 9 and the coolant temperature sensor 11; and produce outputs to the fuel injector 7 and spark plug 12.
Japanese Patent Publication JA-P-56-32451 discloses an arrangement which has been proposed for obviating the deterioration in the emission control and engine performance characteristics which inevitably tend to occur in the above disclosed type of engine when the fuel being fed to the engine changes from a standard type to either so called heavy and light types.
The above mentioned deterioration in emission control is particularly evident in the case a three-way type catalytic converter is being used with the engine.
With this type of arrangement relatively small changes from the target air-fuel ratio have a large effect on the emission concentration level (viz., the concentrations of HC, CO, NOx). For example, as will be appreciated from FIG. 2, in order to maintain the conversion rate above 80% it is necessary to control the air-fuel ratio within a narrow range W wherein the value A is approxiately the stochiometric air-fuel ratio.
Accordingly, the arrangement disclosed in the above mentioned JA-P-56-32451 has attempted to provide the necessary control by including a so called "L - jetronic" type fuel injection control system in an engine system of the nature shown in FIG. 1. In this type of injection system the pressure of the fuel supplied to the injectors is kept constant and the amount of fuel controlled by varying the width of the control signal pulse which is applied to the solenoids of the injectors.
In accordance with this system the injection control pulse width is calculated using the following equation: EQU Ti=Tp(K.times.Qa/N.times.alpha+Ts (1)
wherein:
Tp is the basic injection control width;
Qa is the amount of air sensed being inducted into the engine by an air flow meter 5 or similar device;
N is the rotational speed of the engine;
alpha is a feed-back correction factor which compensates for the delay between the actual combustion and the sensing of the air-fuel ratio of the combusted air-fuel mixture in the exhaust gases; and
Ts is the rise time of the injector.
However, with this type of system when the fuel changes from a standard type to a less volatile type or so called heavy fuel and the engine is subject to acceleration (transient operation), the air-fuel ratio deviates from the target value and the mixture becomes excessively lean. This reduces the drivability of the engine and induces an increase in the emission level.
The reason for this is that the volatilization characterstics in the induction passage is dependent on the content of the fuel and the manner in which it distills into a gaseous fraction and a liquid fraction.
The gaseous fraction of course flows to the combustion chamber much faster than the liquid fraction most of which runs along the walls of the passage.
Accordingly, with heavy fuel wherein the volatile faction is lower than that of regular fuel, the gaseous faction reduces and wall running faction increases as compared with regular fuel.
During steady state operation this distribution essentially no effect on the air-fuel ratio of the air-fuel mixture formed in the combustion chamber or chambers of the engine. However, during vehicle acceleration while the same amount of fuel is injected as in the case wherein regular fuel is used, in the case of heavy fuel, as the wall fraction increases and the gaseous fraction decreases, a longer period of time is required for the injected volume to actually reach and enter the combustion chamber. Thus, the amount of fuel which is permitted to enter the combustion chamber for the instant engine speed and valve timing is reduce due to the increased relatively slow moving wall running fraction.
Conversely, in the event that alcohol or the like is mixed with the fuel or the volatile fraction contained in the fuel increase, the amount gassified fuel as compared with the liquid wall running fraction increases and during acceleration, the amount of fuel actually entering the combustion chamber increases as compared with the case wherein regular fuel is used.
This of course tends to produce a rich mixture which invites increased HC emissions, increased fuel consumption and the like. Of course during steady state operation the effect of light fuel on the air-fuel ratio is essentially non-existant similar to the heavy fuel case.
In certain instances it is possible in the event that heavy fuel is commonly used (for example North America) to shift the target fuel air-fuel ratio from point A toward point B and thus increase the richness of the same in a manner which enables the air-fuel ratio to maintained within the desired limits during both steady state and transitional state operations.
However, if with the above mentioned modified target air-fuel ratio setting, the engine is operated on a fuel having a higher volatile more readily gassifiable fraction, the air-fuel mixture becomes excessively rich during transitional operation such as acceleration and deteriorates the emission control.
The reason for this is that in the case of light fuels which contain relatively large amount of readily volatile material, it is necessary, in order to achieve the same compromise achieved with heavy fuel, to shift the target air-fuel ratio toward point C.
In order to overcome this dilemma it has been proposed to utilize an air-fuel ratio sensor which is capable of produce a response over a relatively wide range of air-fuel ratio as different from commonly used oxygen sensors which exhibit a sudden and marked change in output over a very narrow range the center of which is the stoichiometric air-fuel ratio; and use this sensor in feed-back control loop a manner which enables it to be determined if the air-fuel ratio becomes rich or lean during transitional modes of operation and to shift the target air-fuel ratio in accordance with the outcome of this monitoring. Viz., if the air-fuel ratio is sensed as becomming lean during transitional mode of operation it can be assumed that the fuel being supplied is of the heavy type, and the target air-fuel ratio should be shifted from point A toward point B (FIG. 2) and vice versa.
However, in the event that the calculations necessary for the above mentioned control are carried out in commonly used mircoprocessors, they tend to occupy the CPU of the device to the degree that other calculations which are inevitably required to be concurrently carried out on a cycle by cycle basis, such as ignition timing, fuel injection and the like, cannot be successfully completed. Accordingly, it is necessary to use a high speed processor or a plurality of convential types which are suitably interconnected in a manner which emulates parallel processing or the like.
In either case the cost of the control system is undersirably increased.